Process and apparatus for coating web supports

ABSTRACT

A PROCESS FOR COATING WEB SUPPORTS WITH VISCOUS SOLUTIONS IN WHICH THE SOLUTION IS APPLIED TO THE WEB SUPPORT FROM A COATING CHAMBER THROUGH A NARROW GAP BETWEEN THE EXIT SIDE OF THE CHAMBER AND THE SUPPORT, THE GAP OPENING BEING SEVERAL TIMES LARGER THAN THE THICKNESS OF THE APPLIED SOLUTION LAYER. AN ADJUSTABLE GAS PRESSURE IN EXCESS OF ATMOSPHERIC PRESSURE IS APPLIED AGAINST THE COATING SOLUTIONS DIRECTLY ADJACENT THE OUTER SIDE OF THE COATING CHAMBER WHILE MAINTAINING ATMOSPHERIC PRESSURE ON THE ENTRY SIDE OF THE COATING CHAMBER.

July 11, 1972 K. BROWATZKI ET AL 3,676,178

PROCESS AND APPARATUS FOR COATING WEB SUPPORTS Filed March 17, 1970 INVENTOK KURT BROWATZYLl, HANS GREF, PETER HERZHOFF, FRITZ MAUS, STEPHEN E1322,

GUSTAV SCHAUM, WOLFGANG SCHWEICHER, WILLI WASSER- United States Patent 3,676,178 PROCESS AND APPARATUS FOR COATING WEB SUPPORTS Kurt Browatzki, Opladen, Hans Gref, Cologne, Peter Herzhofr, Leverkuseu, Fritz Mans, Cologne, and Stephan Platz, Gustav Schaum, Wolfgang Schwercher, and Willi Wasser, Leverkusen, Germany, assignors to Agfa-Gevaert Aktiengesellschaft, Leverkusen, Ger- Filed Mar. 17, 1970, Ser. No. 20,227 Claims priority, application Germany, Apr. 25, 1969, P 19 21 644.7 Int. Cl. C23c 13/08 US. Cl. 117-34 2 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to a process and apparatus for coating web supports with viscous solutions, preferably with photographic emulsions, in which the web support is coated in a closed coating chamber and the end of the chamber at which the emulsion leaves is formed by a gap and the layer is formed by an adjustable pressure difierence between the entry and exit sides of the gap, which pressure dilference acts in the opposite direction to that of action which moves the web support past the casting device, the lower pressure being at the entry side.

Coating apparatuses are known in which the web which is to be coated is guided over web guide rollers, part of the circumference of at least one web guide roller dipping into a container for the coating solution. It is also known to adjust the thickness of the emulsion layer by means of a stream of air directed on to the coated web (air brush or air knife process). In this process, the stream of air removes the excess emulsion from the coated web and the layer is smoothed.

To produce a uniform stream of air, an air nozzle in the form of a slot extending over the whole width of the web is provided. The air nozzle is arranged so that the air stream meets the layer within the angular range in which the coated web is still supported on the web guide roller (e.g. British patent specification No. 901,683).

A disadvantage of this process is that splashes may easily occur and that the air stream must be kept very constant and free from turbulence in order to ensure formation of a perfect layer.

Furthermore, processes are known in which the web support is coated in a closed coating chamber and the end of the chamber at which the emulsion leaves is closed a small gap being left between the exit of the chamber and the web support, and in which the layer is formed by an adjustable pressure difierence between the entry and exit sides of the gap, which pressure dilference acts in the opposite direction to that of the moving web support, the lower pressure being at the entry side. The required pressure difiereuce is established by reducing the pressure in the chamber below the pressure of the surrounding atmosphere, for example by evacuation. A control device 3,676,178 Patented July 11, 1972 "ice keeps the level of the coating solution in the container constant.

The disadvantage of this process is that the coating chamber must be very efiiciently sealed off from the atmosphere because bubbles would be carried into the coating solution with the inflowing air at any leakage points and might then settle on the web and produce defects in the coating. Another disadvantage is that at least one air lock barrier must be provided at the entry point of the coating apparatus.

It is the object of the present invention to provide a coating process for use at high coating speeds with which a uniform coating can be applied over the whole surface of the web and in which the thickness of the coating is reproducible within narrow tolerances and can be rapidly adjusted. This problem is solved according to the invention by producing the necessary pressure difference by means of excessive pressure at the exit side of the gap. In the process of the invention, the excessive pressure can be produced either dynamically or statically. To produce the excessive pressure dynamically, an air nozzle arranged directly at the exit side of the gap and extending over the whole width of the web blows a stream of air perpendicularly on to the coated support. The pressure difference is then equal to the dynamic pressure of the air stream with respect to atmospheric pressure. In contrast to the air-brush process described above, the air nozzle in this case is situated directly at the exit side of the gap. In this way, application of the coating is achieved without any backflow. In the air-brush process, no pressure head can build up owing to the large distance of the air nozzle from the exit of the gap.

In another embodiment of the invention, the excessive pressure is produced statically in an excess pressure chamber directly adjacent to the outlet end of the gap by supplying air under pressure from a compressed air system into the excess pressure chamber. The compressed air chamber is connected to the outside by at least one lock chamber.

The excess pressure can be adjusted by a control circuit depending on the flow properties of the coating solution.

The main advantages achieved by the invention lie in the fact that a uniform application of coating over the whole surface of the web can be achieved at high coating speeds with a technically simple construction, and the thickness of the coating can be reproducibly and rapidly adjusted within narrow tolerances.

The process of the invention will now be explained more fully with the aid of a drawing representing two embodiments of the apparatus of the invention. In the drawing:

FIG. 1 shows an apparatus for dynamic production of the excess pressure; and

FIG. 2 shows an apparatus for static production of the excess pressure.

In the apparatus according to 'FIG. 1 the web 11 which is to be coated is guided by the web guide roller 12, passes through the rear closure 13 of the coating chamher and is coated with emulsion in the chamber 14. The wetted Web then passes through a gap 16 and then leaves the coating apparatus. The moving web acts as a pump which draws emulsion out of the container. Directly adjacent to the exit side of the gap 16, there is a nozzle in the form of a slot extending over the whole width of the web, through which a stream of air or steam is blown on to the exit side of the gap 16. The dynamic pressure head of the air stream then acts against the pumping action of the moving web. The dynamic pressure of the air and hence the pressure difference between the exit side of the gap 16 and the chamber 14 determine the thickness of the coating 17 formed on the web.

In the apparatus according to FIG. 2, the web 21 is again guided by the roller 22, passes through the rear closure 23 of the coating apparatus and pumps the emulsion out of the container 24.

In the chamber 26 which is directly adjacent to the exit side of the gap 25 and connected to the outside by lock chambers 28, a stream of air or steam is injected so that the pressure produced is greater than the pressure in the container 24. The leakage losses are constantly compensated by pumping in more air or steam. The excess pressure acts in the opposite direction to that of the pumping action of the web, and the pressure difierence between the chamber 26 and the container 24 again substantially determines the thickness of the coating 27 formed.

The supply of air or steam to cover the leakage losses may be directly fed into the excess pressure chamber 26 from the conduit 29 as sketched in FIG. 2, but in order to achieve a very uniform excess pressure in the excess pressure chamber 26, it is preferable to connect the air or steam supply to one of the lock chambers 28.

We claim:

1. YA process for coating moving web supports with viscous solutions which comprises issuing a viscous solution from a closed coating chamber through a narrow 25 gap between the exit surface of the chamber and a moving web which acts as a support, coating the moving web support with a layer of the viscous solution so that the gap opening is several times larger than the thickness of the applied layer, providing an adjustable pressure difference against the viscous solution at the gap between the side of the coating chamber at which the web enters the gap and the side of the coating chamber from which web leaves the gap, and applying on the viscous solution directly adjacent the outer side a gas pressure in excess of atmospheric pressure while maintaining atmospheric pressure on the entry side of the exit side.

2. In a process as claimed in claim 1, the step of applying the gaseous pressure by a gas under superatmospheric pressure in a chamber.

References Cited UNITED STATES PATENTS 3,503,370 3/1970 Ishiwata et al 117--34 X 3,152,915 10/1-964 Kraus 117-102 LX 2,681,294 6/1954 Beguin 11850 2,733,653 21/1956 Mullen 11863 X 3,038,441 6/1962 lWamsley 118-63 3,220,877 11/1965 Johnson 117-10'2 LX 3,141,194 7/1964 Jester 118-63 X 2,714,745 8/1955 Kenyon 1185O X WILLIAM D. MARTIN, Primary Examiner W. R. TRENOR, Assistant Examiner US. Cl. X.R. 

